Reference picture signaling

ABSTRACT

An encoded representation of a picture of a video stream is decoded by retrieving buffer description from the encoded representation. The buffer description information is used to determine at least one picture identifier identifying a respective reference picture as decoding reference for the picture. A decoded picture buffer is updated based on the determined picture identifier. The encoded representation of the picture itself comprises the information needed by a decoder to identify the reference pictures required to decode the encoded representation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.17/672,561, filed on Feb. 15, 2022, which is a continuation of U.S.patent application Ser. No. 16/885,103, filed on May 27, 2020 (now U.S.Pat. No. 11,265,576), which is a continuation of U.S. patent applicationSer. No. 16/440,274, filed on Jun. 13, 2019 (now U.S. Pat. No.10,708,618), which is a continuation of U.S. patent application Ser. No.16/048,556, filed on Jul. 30, 2018 (now U.S. Pat. No. 10,368,088), whichis a continuation of U.S. patent application Ser. No. 15/718,374, filedon Sep. 28, 2017 (now U.S. Pat. No. 10,063,882), which is a continuationof U.S. patent application Ser. No. 15/299,997, filed on Oct. 21, 2016(now U.S. Pat. No. 9,807,418), which is a continuation of U.S. patentapplication Ser. No. 13/508,442, filed on Apr. 25, 2013 (now U.S. Pat.No. 9,706,223), which is a 35 U.S.C. § 371 national stage application ofPCT International Application No. PCT/SE2012/050439, filed on Apr. 26,2012, which claims the benefit of priority from U.S. ProvisionalApplication No. 61/503,019, filed Jun. 30, 2011, the entire contents ofeach of which are incorporated by reference herein as if set forth intheir entireties.

TECHNICAL FIELD

The embodiments generally relate to reference picture management inconnection with video encoding and decoding, and in particular toreference picture signaling and buffer management.

BACKGROUND

H.264, also referred to as Moving Picture Experts Group-4 (MPEG-4)Advanced Video Coding (AVC), is the state of the art video codingstandard. It consists of a block based hybrid video coding scheme thatexploits temporal and spatial prediction.

High Efficiency Video Coding (HEVC) is a new video coding standardcurrently being developed in Joint Collaborative Team-Video Coding(JCT-VC). JCT-VC is a collaborative project between MPEG andInternational Telecommunication Union Telecommunication standardizationsector (ITU-T). Currently, a Working Draft (WD) is defined that includeslarge macroblocks (abbreviated LCUs for Largest Coding Units) and anumber of other new tools and is considerably more efficient thanH.264/AVC.

At a receiver a decoder receives a bit stream representing pictures,i.e. video data packets of compressed data. The compressed datacomprises payload and control information. The control informationcomprises e.g. information of which reference pictures should be storedin a reference picture buffer, also referred to as a decoded picturebuffer (DPB). This information is a relative reference to previousreceived pictures. Further, the decoder decodes the received bit streamand displays the decoded picture. In addition, the decoded pictures arestored in the decoded picture buffer according to the controlinformation. These stored reference pictures are used by the decoderwhen decoding subsequent pictures.

A working assumption for the processes of decoded picture bufferoperations in the working draft of HEVC is that they will be inheritedfrom H.264/AVC to a very large extent. A simplified flow chart of thescheme as it is designed in H.264/AVC is shown in FIG. 1 .

Before the actual decoding of a picture, the frame_num in the sliceheader is parsed to detect a possible gap in frame_num if SequenceParameter Set (SPS) syntax element gaps_in_frame_num_value_allowed_flagis 1. The frame_num indicates the decoding order. If a gap in frame_numis detected, “non-existing” frames are created and inserted into thedecoded picture buffer (DPB).

Regardless of whether there was a gap in frame_num or not the next stepis the actual decoding of the current picture. If the slice headers ofthe picture contain Memory Management Control Operations (MMCO)commands, adaptive memory control process is applied after decoding ofthe picture to obtain relative reference to the pictures to be stored inthe decoded picture buffer; otherwise a sliding window process isapplied to obtain relative reference to the pictures to be stored in thedecoded picture buffer. As a final step, the “bumping” process isapplied to deliver the pictures in correct order.

A problem with H.264/AVC is its vulnerability to losses of pictures thatcontains MMCO of type 2, 3, 4, 5 or 6 as described in Table 1 below.

TABLE 1 Memory management control operation values for H.264/AVCmemory_manage- ment_control_operation Memory Management ControlOperation 0 End memory_management_control_operation syntax element loop1 Mark a short-term reference picture as “unused for reference” 2 Mark along-term reference picture as “unused for reference” 3 Mark ashort-term reference picture as “used for long-term reference” andassign a long- term frame index to it 4 Specify the maximum long-termframe index and mark all long-term reference pictures having long-termframe indices greater than the maximum value as “unused for reference” 5Mark all reference pictures as “unused for reference” and set theMaxLongTermFrameIdx variable to “no long-term frame indices” 6 Mark thecurrent picture as “used for long- term reference” and assign along-term frame index to it

Loss of a picture that does not contain MMCO, or a picture that containsMMCO of type 0 or 1, is of course severe to the decoding process. Pixelvalues of the lost picture will not be available and may affect futurepictures for a long period of time due to incorrect inter prediction.There is also a risk that reference picture lists for a few picturesfollowing the lost picture will be wrong, for example if the lostpicture contained MMCO that marked one short-term reference picture as“unused for reference” that otherwise would have been included in thereference picture list of the following picture. However, the decodingprocess can generally recover such a loss through usage of constrainedintra blocks, intra slices or by other means.

But if a picture containing MMCO of type 2, 3, 4, 5 or 6 is lost thereis a risk that the number of long term pictures in the DPB is differentfrom what it would have been if the picture was received, resulting inan “incorrect” sliding window process for all the following pictures.That is, the encoder and decoder will contain a different number ofshort-term pictures resulting in out-of-sync behavior of the slidingwindow process. This loss cannot be recovered through usage ofconstrained intra blocks, intra slices or similar techniques (not evenan open Group Of Picture (GOP) Intra picture). The only way to ensurerecovery from such a loss is through an Instantaneous Decoder Refresh(IDR) picture or through an MMCO that cancels the effect of the lostMMCO. What makes the situation even worse is that a decoder will notnecessarily know that the sliding window process is out-of-sync and thuscannot report the problem to the encoder or request an IDR picture evenin applications where a feedback channel is available.

One way to reduce the risk of loosing important MMCO information is touse dec_ref_pic_marking_repetition Supplementary Enhancement Information(SEI) messages. However the encoder will not know if the decoder iscapable of making use of dec_ref_pic_marking_repetition SEI messages.Further, there is a risk that the dec_ref_pic_marking_repetition SEImessage is also lost.

There is, thus, a need for an efficient reference picture signaling andbuffer management that do not suffer from the shortcomings andlimitations of prior art solutions.

SUMMARY

It is a general objective to provide an efficient reference picturesignaling and buffer management in connection with video encoding anddecoding.

This and other objectives are met by embodiments disclosed herein.

An aspect of the embodiments relates to a method of decoding an encodedrepresentation of a picture of a video stream of multiple pictures. Themethod comprises retrieving buffer description information defining atleast one reference picture from the encoded representation of thepicture. At least one picture identifier identifying a respectivereference picture of the at least one reference picture is determinedbased on the buffer description information. The respective referencepicture is to be used as decoding reference for the picture. A decodedpicture buffer is updated based on the determined at least one pictureidentifier.

A related aspect of the embodiments defines a decoder configured todecode an encoded representation of a picture of a video stream ofmultiple pictures. The decoder comprises a data retriever configured toretrieve buffer description information defining at least one referencepicture from the encoded representation of the picture. A pictureidentifier determiner of the decoder is configured to determine, basedon the buffer description information, at least one picture identifieridentifying a respective reference picture of the at least one referencepicture as decoding reference for the picture. The decoder alsocomprises a buffer manager configured to update a decoded picture bufferbased on the at least one picture identifier.

Another related aspect of the embodiments defines a decoder comprisingan input section configured to receive encoded representations ofmultiple pictures of a video stream. The decoder also comprises aprocessor configured to process code means of a computer program storedin a memory. The code means causes, when run on the processor, theprocessor to retrieve buffer description information defining at leastone reference picture from an encoded representation of a picture. Theprocessor is also caused to determine, based on the buffer descriptioninformation, at least one picture identifier identifying a respectivereference picture of the at least one reference picture as decodingreference for the picture. The code means further causes the processorto update a decoded picture buffer based on the at least one pictureidentifier. An output section of the decoder is configured to outputdecoded pictures of the video stream.

Another aspect of the embodiments relates to a method of encoding apicture of a video stream of multiple pictures. The method comprisesdetermining, for the picture, at least one reference picture of themultiple pictures as encoding reference for the picture. The method alsocomprises providing, for each reference picture, a picture identifieridentifying the reference picture. Buffer description informationdefining the at least one reference picture is generated based on the atleast one picture identifier. The buffer description information isinserted in an encoded representation of the picture.

A related aspect of the embodiments defines an encoder configured toencode a picture of a video stream of multiple pictures. The encodercomprises a reference picture determiner configured to determine, forthe picture, at least one reference picture of the video stream asencoding reference for the picture. The encoder also comprises a pictureidentifier provider configured to provide, for each reference picture, apicture identifier identifying the reference picture. A bufferdescription information generator is configured to generate bufferdescription information based on the at least one reference pictureidentifier. The buffer description information is inserted by a datainserter in an encoded representation of the picture.

Another related aspect of the embodiments defines an encoder comprisingan input section configured to receive multiple pictures of a videostream and a processor configured to process code means of a computerprogram stored in a memory. The code means causes, when run on theprocessor, the processor to determine, for a picture of the videostream, at least one reference picture of the video stream as encodingreference for the picture. The processor is also caused to provide, foreach reference picture, a picture identifier identifying the referencepicture and generate buffer description information based on the atleast one picture identifier. The code means further causes theprocessor to insert the buffer description information in an encodedrepresentation of the picture. The encoder further comprises an outputsection configured to output encoded representations of pictures.

In clear contrast to the prior art solutions in which correct referencepicture management is dependent on that previously encoded pictures havebeen correctly received and decoded, the embodiments provide bufferdescription information that is used for reference pictures in anabsolute and explicit way instead of a relative or implicit way. Thus,the encoded representation of a picture contains the information aboutwhich reference pictures to use for reference during decodingindependent of the encoded representations of previous pictures in thevideo stream.

The embodiments thereby make reference picture management and signalingless vulnerable to errors since the decoder will only have to rely oninformation contained in the current picture for the reference picturemanagement of the current picture instead of relying on correctlydelivered and interpreted buffer operations in previous pictures.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, maybest be understood by making reference to the following descriptiontaken together with the accompanying drawings, in which:

FIG. 1 is a simplified flow chart of the H.264/AVC reference bufferscheme;

FIG. 2 is an example of a coding structure with two temporal layers;

FIG. 3 is a flow chart of a method of encoding a picture according to anembodiment;

FIG. 4 is a video stream of multiple pictures according to anembodiment;

FIG. 5 is an encoded representation of a picture according to anembodiment;

FIG. 6 is a flow chart of an additional, optional step of the method inFIG. 3 ;

FIG. 7 is a flow chart of additional, optional steps of the method inFIG. 3 and an embodiment of generating buffer description information inFIG. 3 ;

FIG. 8 is an example of a coding structure with (sub)GOP size 8;

FIG. 9 is a flow chart of a method of decoding an encoded representationof a picture according to an embodiment;

FIG. 10 is a flow chart of an embodiment of determining pictureidentifier in FIG. 9 ;

FIG. 11 is a flow chart of an embodiment of retrieving bufferdescription information in FIG. 9 ;

FIG. 12 is a flow chart of another embodiment of determining pictureidentifier in FIG. 9 ;

FIG. 13 is a flow chart of additional, optional step of the method inFIG. 9 ;

FIG. 14 is a flow chart of an additional, optional step of the method inFIG. 9 ;

FIG. 15 is an example of a coding structure;

FIG. 16 is a simplified flow chart of a reference buffer schemeaccording to an embodiment;

FIG. 17 is a schematic block diagram of a transmitter according to anembodiment;

FIG. 18 is a schematic block diagram of an encoder according to anembodiment;

FIG. 19 is a schematic block diagram of an encoder according to anotherembodiment;

FIG. 20 is a schematic block diagram of a receiver according to anembodiment;

FIG. 21 is a schematic block diagram of a decoder according to anembodiment; and

FIG. 22 is a schematic block diagram of a decoder according to anotherembodiment.

DETAILED DESCRIPTION

Throughout the drawings, the same reference numbers are used for similaror corresponding elements.

The present embodiments generally relate to encoding and decoding ofpictures, also referred to as frames in the art, of a video stream. Inparticular, the embodiments relate to management of reference picturesin connection with video encoding and decoding and the signaling of suchreference pictures from the encoder to the decoder.

Video encoding, such as represented by H.264/MPEG-4 AVC and HEVC,utilizes reference pictures as predictions or references for theencoding and decoding of pixel data of a current picture. This isgenerally referred to as inter coding in the art where a picture isencoded and decoded relative to such reference pictures. In order to beable to decode an encoded picture, the decoder thereby has to know whichreference pictures to use for the current encoded picture and has tohave access to these reference pictures. Generally, the decoder uses adecoded picture buffer (DPB), also denoted reference picture bufferherein, for storing the reference pictures. It is then important thatthe reference pictures stored in the decoded picture buffer are indeedthe correct reference pictures when decoding an encoded pictureotherwise the decoder will use wrong reference pictures during thedecoding process causing a degradation of the quality of the presentedvideo.

The prior art techniques may suffer from problems with regard to usingincorrect reference pictures when a picture carrying MMCO information isunintentionally lost, which was discussed in the background section.This problem of the prior art can be illustrated by the followingH.264-implemented example. Assume that the decoded picture buffer storesthree short term pictures with picture identifiers 300, 302 and 303 andtwo long term pictures with picture identifiers 0 and 3. The encodermight then generate a new encoded picture with a MMCO type 2 commandstating that the long term picture 0 should be unused for reference. Ifthis encoded picture would have been correctly received at the decoderthe long term picture 0 would have been marked as unused for referenceand the reference picture list would have been {300, 302, 303, 3}.However, if the encoded picture with the MMCO type 2 command is lost,the decoder is not informed that the long term picture 0 should bemarked as unused for reference and the reference picture list istherefore instead {300, 302, 303, 0, 3}. If a next encoded picturereceived at the decoder comprises information that reference picture atposition 3 in the reference picture list is to be used as prediction fora macroblock in the picture there will be a problem if the MMCO type 2command is lost. If the MMCO type 2 command had been correctly receivedat the decoder, the reference picture at position 3 in the referencepicture list would correspond to the long term picture 3 as thisreference picture occupies position 3 (if starting with 0) in thereference picture list. However, with a lost MMCO type 2 commandposition 3 in the reference picture list is instead occupied by the longterm picture 0. This means that pixel data from the long term picture 0will be used as prediction basis instead of the correct pixel data fromthe long term picture identifier 3.

Thus, the prior art solution has a problem that correct referencepicture management is dependant on that previously decoded pictures havebeen correctly received and decoded.

The present embodiments do not have these problems of the prior arttechniques by using a fundamentally different approach for signalingreference pictures as compared to the prior art. The present embodimentsinstead specify which decoded pictures to be used for reference picturesin an absolute or explicit way instead of a relative or implicit way.Another way to put it is that the encoded representation, i.e. thebitstream, for a current picture contains the information about whatpictures to use for reference, i.e. reference pictures, independent ofthe encoded representations of previous pictures. It can therefore besaid that the logical responsibility for maintaining correct decodedpicture buffer is moved from the decoder to the bitstream. One way tolook at it is to say that the information about what reference picturesto use for inter prediction and motion vector prediction for a pictureis included in the control information of the picture. Hence, the stateof the decoded picture buffer is signaled for every picture that isencoded and decoded relative to other pictures.

According to an aspect of the embodiments a method is provided thatcreates buffer description information, e.g. a data structure, such as ageneral table with absolute information of which pictures to be used forreference pictures, i.e. stored in a decoded picture buffer (alsoreferred to as a reference picture buffer) to be used for subsequentcoding. At least a part of the buffer description information isinserted in the bitstream encoded by an encoder.

FIG. 3 is a flow chart of a method of encoding a picture according to anembodiment. The method generally starts in step S1 where at least onereference picture of a video stream of multiple pictures is determinedas encoding reference. In an embodiment, step S1 determines one or morereference pictures that are used as encoding reference for a currentpicture to be encoded. Hence, the pixel data of the current picture isthen encoded with reference to the one or more reference pictures.Alternatively, or in addition, at least one reference picture determinedin step S1 could be used as encoding reference for a subsequent pictureof the video stream, i.e. a picture to be encoded and decoded after thecurrent picture. This subsequent picture is thereby after the currentpicture according to a decoding order (and encoding order). In aparticular embodiment S1 determines, for the current picture, anyreference picture of the video stream as encoding reference for thecurrent picture and any reference picture of the video stream asencoding reference for the subsequent picture. Hence, in a particularembodiment step S1 determines all reference pictures that are prior tothe current picture in decoding order and that may be used for interprediction for the current picture or any picture following the currentpicture according to the decoding order.

FIG. 4 schematically illustrates this concept by showing a video stream1 of multiple pictures 10, 40, 42, 50. A current picture 10 may compriseone or more slices 20, 22 comprising pixel blocks 30, such asmacroblocks, also referred to as treeblocks, or coding units, to bedecoded. The arrows below the pictures 10, 40, 42, 50 indicate thedecoding relationship. The current picture 10 is decoded in relation toa previous reference picture 40 and a subsequent reference picture 42.The preceding reference picture 40 is preceding and the subsequentreference picture 42 is subsequent with regard to the current picture 10according to the output order but both are preceding the current picture10 according to the decoding order. This subsequent reference picture 42is furthermore used as reference picture for a subsequent picture 50 inthe video stream 1. Hence, in a particular embodiment step S1 coulddetermine reference pictures as pictures 40, 42 in FIG. 4 .

A next step S2 of FIG. 3 provides a respective picture identifier foreach reference picture of the at least one reference picture determinedin step S1. The picture identifier is used, possibly together with otherdata, to unambiguously identify a reference picture. Hence, the pictureidentifier can be regarded as an absolute reference to pictures to beused as reference pictures. This means that it is possible to correctlyidentify the relevant reference picture given its picture identifier andoptionally the other data.

There are various alternatives available that could be used as pictureidentifier according to the embodiments. For instance, the pictureidentifier could be the decoding order number, the display order number,the output order number or a combination of display order number and anadditional identifier or indeed any other information that can be usedto unambiguously identify the picture.

Examples of such picture identifiers include Picture Order Count (POC),frame number (frame_num) or POC and an additional identifier (additionalpicture_id).

In a particular embodiment, the actual value of the picture identifieris used together with additional information or other data, such as theposition of the picture identifier in buffer description informationthat is generated in step S3 to unambiguously identify the relevantreference picture. Hence, the buffer description identified or obtainedby the buffer description information enables an unambiguouslyidentification of the relevant reference picture(s). In an embodiment,the picture identifier itself, such as POC or POC plus an additionalidentifier, can be used to unambiguously identify the reference picture.

Unambiguously identify a reference picture is used herein to denote thatthe picture identifier itself or the picture identifier together withother information in the buffer description information, such as theorder at which the buffer description information defines the pictureidentifiers, is used to explicitly identify a reference picture. Hence,given the picture identifier or the picture identifier and the otherinformation enables identification of the relevant reference pictureamong the pictures of the video stream.

In a particular embodiment of step S1, the total number of referencepictures determined for the current picture could be restricted by aparameter that can be signaled from the encoder to the decoder, such asa parameter denoted max_num_ref_frames.

Step S2 is preferably performed for each reference picture determined instep S1, which is schematically illustrated by the line L1.

The picture identifier(s) provided in step S2 could be read from headerportions of the reference picture(s) determined in step S1 or otherwiseretrieved from the data representing the reference picture(s) from stepS1.

The next step S3 generates information of a buffer description, alsoreferred to as Reference Picture Set (RPS). This information is denotedbuffer description information herein. The buffer descriptioninformation is generated based on the picture identifier(s) provided instep S2. This buffer description information defines, preferablyunambiguously defines, the at least one reference picture determined instep S1. Hence, it is possible to derive the respective pictureidentifier of the at least one reference picture from the bufferdescription information.

The generated buffer description information is inserted in step S4 inan encoded representation of the current picture. Hence, the encodedpicture therefore carries the buffer description information that can beused at the decoder to define and identify the reference pictures thatare needed to decode the current picture and/or any subsequent pictureof the video stream.

The buffer description information is therefore provided in controlinformation of the encoded picture provided from the encoder to adecoder. At a minimum, the buffer description information containsinformation needed by the decoder to identify the reference pictures tobe used in the decoded picture buffer.

The buffer description information can therefore be regarded asinformation identifying a buffer description that is a set of referencepictures associated with a current picture. It consists of all referencepictures that are prior to the current picture in decoding order andthat may be used for inter prediction of the current picture or anypicture following the current picture in decoding order.

In an embodiment, the buffer description information contains or definesinformation about each picture that is used by the decoder in thedecoding process even if the picture is not used for inter prediction ormotion vector prediction or any other prediction. Such information couldinclude, but is not limited to, decoding order, display order,temporally layer information and view information.

As mentioned above, the number of reference pictures that can besignaled by the buffer description information could be restricted bythe parameter max_num_ref_frames. However, the buffer descriptioninformation may define fewer than this maximum number of pictures, inwhich case the remaining ones are interpreted as “empty”.

The method of FIG. 3 with steps S1 to S4 is preferably performed foreach picture in the video stream except any Instantaneous DecoderRefresh (IDR) picture that cause a decoded picture buffer refresh andthereby does not need any buffer description information, which isschematically illustrated by the line L2. Hence, each encodedrepresentation generated by the encoder preferably carries bufferdescription information defining the reference pictures used forencoding and decoding the current picture and/or any subsequent picturein the video stream.

This approach of the embodiments provides a significant advantage overthe relative reference picture signaling of the prior art using MMCOcommand. The explicit signaling of reference pictures through the bufferdescription information in each encoded representation of the picturesin the video stream makes reference picture management less vulnerableto errors and increases the error robustness level of the decoder. Thus,instead of relying on correctly delivered and interpreted bufferoperations in previous pictures, the decoder will only have to rely oninformation contained in the encoded representation of the currentpicture.

In a particular embodiment, the buffer description information insertedin the encoded representation of the picture in step S4 is in fact thebuffer description itself. Thus, the buffer description information thencomprises a listing of the picture identifier(s) provided in step S2 ordata allowing calculation of the picture identifier(s) provided in stepS2. This latter case will be further described below in connection withFIG. 6 .

For instance, the buffer description could define a list with pictureidentifiers 3, 5 and 6 as the reference pictures for a current picture.The buffer description information inserted in the encodedrepresentation in step S4 would then include these picture identifiers3, 5 and 6.

An alternative approach that is generally more bit efficient, i.e.generally requires fewer number of bits or symbols for defining thepicture identifiers, is to signal the reference picture properties, i.e.picture identifiers, relative to the value of these properties assignaled for the current picture. For instance, if the current picturehas a picture identifier 7 the list of reference pictures withidentifiers 3, 5 and 6 could be defined as −1, −2 and −4, whichtypically can be represented by fewer bits as compared to 3, 5 and 6, inparticular if variable length coding is employed for the pictureidentifiers.

FIG. 6 schematically illustrates this approach. The method continuesfrom step S2 of FIG. 3 . In a next step S10 a difference is calculatedfor each picture identifier provided in step S2, between the pictureidentifier and a picture identifier identifying the current picture. Asa result of this calculation a difference or delta identifier or valueis obtained. The method then continues to step S3 of FIG. 3 , where thebuffer description information is generated based on the calculateddifference(s) or delta identifier(s).

The buffer description information could therefore in this case includethe delta identifiers −1, −2 and −4 instead of 3, 5 and 6.

In an embodiment, delta display order information or deltaPOC isincluded in the buffer description encoded with a variable length code(VLC). In a particular embodiment deltaPOC is encoded with VLC forabsolute_delta_POC_minus_one and a flag, i.e. a single bit, fordeltaPOC_sign signaled only if number_of_reorder_frames>0, otherwise thesign is inferred to be negative.

In the above embodiments providing an explicit signaling of the pictureidentifiers, either the picture identifiers themselves or the deltaidentifiers, the buffer description information will in fact constitutethe buffer description of the current picture. This buffer descriptioninformation is then inserted in the encoded representation of thepicture.

The buffer description information could be included as controlinformation at a suitable position in the encoded representation. FIG. 5schematically illustrates an example of an encoded representation 60 ofa picture. The encoded representation 60 comprises video payload data 66that represents the encoded pixel data of the pixel blocks in a slice.The encoded representation 60 also comprises a slice header 65 carryingcontrol information. The slice header 65 forms together with the videopayload and a Network Abstraction Layer (NAL) header 64 a NAL unit thatis the entity that is output from an encoder. To this NAL unitadditional headers, such as Real-time Transport Protocol (RTP) header63, User Datagram Protocol (UDP) header 62 and Internet Protocol (IP)header 61, can be added to form a data packet that can be transmittedfrom the encoder to the decoder. This form of packetization of NAL unitsmerely constitute an example in connection with video transport. Otherapproaches of handling NAL units, such as file format, MPEG-2 transportstreams, MPEG-2 program streams, etc. are possible.

The buffer description information could then be included in the sliceheader 65, another picture header or another data structure specified bythe standard to which the encoder and decoder conforms.

In another embodiment the buffer description information inserted in theencoded representation 60 of the picture does not necessarily have to bethe same as the buffer description of the current picture but ratherenables identification and retrieval of the buffer description. Thus, inthis embodiment the buffer description information inserted in theencoded representation 60 of the picture indirectly defines the at leastone reference picture determined in step S1 by pointing towards thebuffer description which carries the picture identifiers or the data,such as delta identifiers, enabling calculation of the pictureidentifiers.

In such a case, the buffer description could be carried by a datastructure associated to the encoded representation 60 of the picture.Examples of such data structures include a Picture Parameter Set (PPS)67 and a Sequence Parameter Set (SPS) 68. The PPS 67 and/or the SPS 68could be directly included in the encoded representation but istypically associated thereto through the inclusion of a PPS identifierand/or SPS identifier in the encoded representation 60. For instance,each slice header 65 could include a PPS identifier notifying which PPS67 to apply for the current picture. The relevant PPS 67 may in turninclude an SPS identifier notifying which SPS 68 to apply for the PPS 67and therefore for the current picture.

The buffer description could then be inserted in the PPS 67 or the SPS68 assigned to the current picture. In such a case, the PPS identifieror SPS identifier that is inserted in the encoded representation 60constitutes the buffer description information that is inserted in theencoded representation 60. This PPS identifier or SPS identifier enablesretrieval of the buffer description that defines the picture identifiersof the reference pictures and the PPS identifier or SPS identifiertherefore indirectly defines the picture identifiers.

PPS 67 and SPS 68 merely constitutes examples of data structuresassociated to encoded representations 60 of pictures and which can beused to carry buffer description information according to theembodiments.

FIG. 7 illustrates an alternative embodiment where one or more bufferdescriptions are signaled in a data structure so that the same bufferdescription can be used for multiple pictures.

The method starts in step S20 where a data structure, such as a table,is generated. The data structure comprises multiple predefined bufferdescriptions each defining at least one reference picture.

Each buffer description of the generated data structure could define thepicture identifiers directly, i.e. include a list of pictureidentifiers. However, such an approach generally requires quite a fewpredefined buffer description in the data structure.

A more efficient approach is to combine the usage of multiple predefinedbuffer descriptions with signaling of delta identifiers as discussed inthe foregoing. In such a case, each predefined buffer descriptioncomprises at least one respective delta identifier, which is used at thedecoder together with the picture identifier of the current picture tocalculate the picture identifier(s) of a predefined buffer description.

Table 1 below illustrates an example of such a data structure with deltaidentifiers that can be used for a video stream as illustrated in FIG. 8. The video stream of FIG. 8 starts with an IDR picture, which is anIntra (I) frame that removes all dependencies to data transmitted priorto the IDR picture, i.e. it marks all reference pictures as “unused forreference”. The IDR picture does not need a buffer description since itempties the decoded picture buffer. The video stream of FIG. 8 is in theform of layered video providing pictures in different temporal layers,identified by temporal identifiers (temporal_id) n, n+1 and n+2 in FIG.8 .

TABLE 1 data structure with predefined buffer descriptions Entry Picturebuffer 1 Picture buffer 2 Picture buffer 3 Picture buffer 4 0 dP: −1tId: 2 dP: −2 tId: 1 dP: −4 tId: 0 dP: −8 tId: 0 1 dP: −1 tId: 0 dP: −2tId: 2 dP: −5 tId: 0 dP: −9 tId: 0 2 dP: −1 tId: 2 dP: −2 tId: 0 dP: −6tId: 0 dP: −10 tId: 0 3 dP: −1 tId: 1 dP: −2 tId: 2 dP: −3 tId: 0 dP: −7tId: 0

Table 1 shows an example of a buffer description table where deltaPOC(dP) and temporal_id (tId) are indicated. The table is constructed usingthe scheme of including the two closest reference pictures(POC(current)−1 and POC(current)−2) and the two from the lowest temporallayer in the range from POC(current)−3 to POC(current)−10.

An example use of this table by the encoder is to, for a picture withPOC=n, signal table entry (n %4), i.e. n modulo 4, to the decoder. Inthis example, the decoded picture buffer consists of four pictures(picture buffer 1 to picture buffer 4). Which pictures depends on thePOC of the current picture and what entry is used. For example ifpicture with POC=7 uses entry 3 the reference pictures in the decodedpicture buffer will consist of pictures with POC 16, 5, 4, 01.

The data structure generated in step S20 is signaled from the encoder tothe decoder. This signaling can be performed according to variousembodiments. The data structure could be carried in the PPS, the SPS, anovel parameter set or in another data structure specified by thestandard to which the encoder and decoder conforms. This isschematically illustrated by step S21 where the data structure isinserted in a PPS or a SPS associated with an encoded representation ofthe video stream. In such a case, step S22 preferably inserts a PPSidentifier or SPS identifier in the encoded representation of thepicture, such in a slice header. This PPS identifier or SPS identifierthen enables identification of the data structure that is available whendecoding the current picture.

The method continues to steps S1 and S2 of FIG. 7 where referencepictures are determined and picture identifiers are provided for thecurrent picture. A next step S23 selects, based on the at least onepicture identifier provided in step S2, a buffer description from thedata structure generated in step S20.

Once such a buffer description, such as an entry in Table 1, has beenselected step S24 generates buffer description information comprising anidentifier, such as entry number, of the selected buffer description.The method continues to step S4 where the buffer description informationis inserted in the encoded representation of the picture.

Thus, in order to specify which buffer description to use for thecurrent picture an identifier can be signaled for the current picture.An example of such an identifier is a non-negative integer signaled inthe slice header(s) of the current picture representing the number ofthe buffer description in the order in which the buffer descriptions aresignaled.

In a typical implementation step S20 is performed once for the videostream or once for a set of multiple pictures of the video stream. Thismeans that a single such data structure is then generated for the videostream or for the set of multiple pictures. The following steps S21 toS24 of FIG. 7 are preferably performed for each picture in the videostream or the set of multiple pictures.

Thus, the data structure, such as table, may be created in the encoderand transmitted to the decoder. In the control information of theencoded bitstream the entry number to be used is provided. By using sucha table the decoder can obtain absolute information of which pictures tobe used as reference pictures by detecting the entry number in thedecoded bitstream and using that entry number to look up the entry inthe table. The entry is then used to determine which pictures to bestored in the decoded picture buffer.

Introducing buffer description entries in, for instance, the SPS reducesthe bit overhead of signaling the buffer descriptions explicitly in theslice header. These buffer descriptions can be used for multipleslices/pictures in the same sequence, i.e. video stream, and thus reducethe number of bits required per picture.

For each buffer description entry in the PPS the delta_POC andtemporal_id of all reference pictures could be present in an embodiment.The optional additional_picture_id is preferably inferred to be 0 forpictures described by an entry in the PPS.

According to a further embodiment, explicit signaling of bufferdescription and reference signaling to an entry in a general datastructure with multiple predefined buffer descriptions, such as an entryin the table above, can be combined. In such a case, these can becombined by the decoder to form a final buffer description for thecurrent picture. One way to combine the explicit signaling and thereference signaling is to join the set of reference pictures describedby explicit signaling with the set of reference pictures described bythe reference signaling to form a joint set of reference pictures.

In such a case, step S3 of FIG. 3 preferably comprises generating thebuffer description information to comprise the identifier of the bufferdescription selected in step S23 and to comprise information defining atleast one reference picture. This information defining at least onereference picture could be the picture identifier of the referencepicture itself or a delta identifier from which the picture identifiercan be calculated.

A picture that is used for reference for a long period of time (longterm reference picture) is preferably indicted by explicit descriptionin the slice headers of the pictures for which it is available forreference. The reason is that it would be inconvenient to include everydistance in POC numbers in the predefined buffer descriptions signaledin the PPS or SPS.

In a particular embodiment, the encoded representation of the picturepreferably comprises a flag to indicate whether explicit signaling ofthe buffer description information and/or implicit signaling of thebuffer description information has been selected for the currentpicture. This flag could, for instance, be included in the slice headerof the encoded representation of the picture or in some other controlinformation field.

In a particular embodiment, a picture 10 can be composed of one ormultiple slices 20, 22 as shown in FIG. 4 . In such a case, a slice 20,22 is an independently decodable portion of the picture 10. In otherwords, an encoded slice can be decoded even if the data of anotherencoded slice of the same picture 10 is lost.

In such an approach step S1 of FIG. 3 preferably determines at least onereference picture of the video stream for each slice in the picture. Itcould then be possible that a first slice will use a first set of one ormore reference pictures and a second slice will use a second set of oneor more reference pictures. The second set could be equal to the firstset or be different from the first set. It is also possible that atleast one of the reference pictures is common for the first set and thesecond set.

Step S2 preferably determines the picture identifiers for all referencepictures determined in step S1 for at least one slice, preferably forall of the slices, in the picture. The buffer description information isthen generated in step S3 based on these picture identifiers and therebydefines the reference pictures. In an embodiment, the buffer descriptiongenerated in step S3 is inserted in a respective slice header of theencoded representation of the picture for each slice. Thus, each sliceheader of the encoded representation of the picture preferably carries arespective instance of the buffer description information. Morepreferably, the buffer description information carried in the sliceheader of a first slice of the picture is the equal to the bufferdescription information carried in the slice header of a second slice ofthe picture.

Video encoding and decoding can also be applied to so-called scalable orlayered video. For instance, temporal scalability is supported inH.264/MPEG-4 AVC and Scalable Video Coding (SVC) through the definitionof subsequences and usage of temporal_id in SVC and insertion of“non-existing” frames. However, in order to support temporalscalability, the pictures in the higher temporal layers are restrictedwhen it comes to usage of MMCO. The encoder is responsive of making surethat the MMCOs in one temporal layer does not affect pictures of lowertemporal layers differently compared to if the temporal layer is droppedand “non-existing” pictures are inserted and sliding window process isapplied.

This imposes restrictions on the encoder in selection of codingstructure and reference picture usage. For instance, consider theexample in FIG. 2 . Assume that the maximum number of reference framesin the decoded picture buffer (max_num_ref_frames) is three even thougheach picture only uses two reference pictures for inter prediction. Thereason is that each picture must hold one extra picture from the othertemporal layer that will be used for inter prediction by the nextpicture.

In order to have picture POC=0 and picture POC=2 available when decodingpicture POC=4, picture POC=3 must have an explicit reference picturemarking command (MMCO) marking picture 1 as unavailable.

However, if temporal layer 1 is removed (for example by a network node)there will be gaps in frame_num for all odd numbered pictures.“Non-existing” pictures will be created for these pictures and slidingwindow process will be applied. That will result in having the“non-existing” picture POC=3 marking picture POC=0 as unavailable. Thus,it will not be available for prediction when picture POC=4 is decoded.Since the encoder cannot make the decoding process be the same for thetwo cases; when all pictures are decoded and when only the lowest layeris decoded; the coding structure example in FIG. 2 cannot be used fortemporal scalability according to prior art.

Thus, prior art solutions have problems with temporal scalability forcertain coding structures since the information about the referencepictures for the current picture is affected by the removal of aprevious picture from the bitstream as the reference picture informationis signaled in a relative way. Using the embodiments, temporalscalability will not be restricted as in the prior since the referencepicture for the current picture is not affected by removal of previouspictures from the bitstream since it is signaled in an absolute way.

In the case of a scalable video stream with the pictures grouped intomultiple layers step S2 of FIG. 3 preferably comprises providing pictureidentifier and temporal layer information identifying a layer of themultiple layers to which the reference picture belongs. The bufferdescription information is then generated in step S3 based on the atleast one picture identifier and the temporal layer information. Thismeans that the buffer description information thereby defines the atleast one picture identifier and the temporal layer information.

For instance, temporal layer information, such as temporal_id, isincluded for each picture in the buffer description signaled usingceil(log 2(max_temporal_layers_minus1)) bits for signaling of thetemporal_id. Temporal scalability is merely an example of multi-layervideo to which the embodiments can be applied. Other types includemulti-layer video where each picture has a picture identifier and a viewidentifier. Further examples of scalability include spatial scalability,signal-to-noise ratio (SNR) scalability, bit-depth scalability andchroma format scalability.

The embodiments imply that temporal down-switching is always possible.Each temporal layer together with the lower layers will constitute asub-sequence. Such sub-sequences need no explicit signaling.

Generally, the encoder is free to select which pictures to include inthe buffer description and may base its selection on any aspect, such asthe closest pictures in display order. Typically, an encoder selects thebuffer description to try to achieve maximum compression while complyingto a set of side constraints. One example of such a constraint is amaximum number of reference pictures due to memory size. Another exampleis that the video stream shall be decodable also when certain codedpictures in the bitstream are removed from the bitstream beforedecoding. A further example is that only pictures that are available forreference in the decoded picture buffer can be selected as referencepictures.

Thus, in an embodiment step S1 of FIG. 3 comprises determining the atleast one reference picture for the current picture by maximizingcompression efficiency of the encoded representation of the picturewhile complying to at least one side constraint. The at least one sideconstraint is then preferably selected from a predefined maximum numberof reference pictures and generating a decodable encoded representationof the video stream also when at least one encoded representation of apicture is removed from the encoded representation of the video stream.

According to another aspect of the embodiments a method is provided inwhich an encoded video or data stream is received, buffer descriptioninformation is detected in the encoded video stream and absolutereference to pictures to be used as reference pictures for decoding acurrent picture or as reference pictures for future decoding isdetermined based on the buffer description information. The pictures tobe used as reference pictures are stored in a decoded picture buffer.

FIG. 9 is a flow chart of a method of decoding an encoded representationof a picture according to an embodiment. The method starts in step S30where buffer description information defining at least one referencepicture is retrieved from the encoded representation of the picture. Thebuffer description information is used in step S31 to determine at leastone picture identifier that, preferably unambiguously, identifies arespective reference picture as decoding reference for the picture. Theat least one reference picture identifier determined in step S31 is usedin step S32 to update a decoded picture buffer.

Once the decoded picture buffer has been updated to comprise the correctreference pictures needed for decoding the picture the method typicallycontinues to step S33 where the picture is decoded based on the encodedrepresentation of the picture and at least one of the reference picturescomprised, present or stored in the decoded picture buffer andidentified based on the buffer description information.

In a preferred embodiment, the decoding step S33 is performed afterupdating the decoded picture buffer as indicated in FIG. 9 .

The updating of the decoded picture buffer in step S32 preferablyimplies that the reference picture identified by the determined pictureidentifier is marked as “used for reference” or marked as “used forprediction” to indicate that this reference pictures is to be used asdecoding reference or prediction for the current picture and/or anysubsequent picture. In a particular embodiment, reference pictures couldbe marked as used for short-term reference or as used for long-termreference.

In a particular embodiment step S30 retrieves the buffer descriptioninformation defining multiple reference pictures from the encodedrepresentation of the picture. In such an embodiment, step S31 coulddetermine, based on the buffer description information, i) at least onepicture identifier identifying a respective reference picture asdecoding reference for the picture and ii) at least one pictureidentifier identifying a respective reference picture as decodingreference for a subsequent, according to a decoding order, picture ofthe video stream. In alternative embodiments, the buffer descriptioninformation identifies one or more picture identifiers of respectivereference picture(s) as decoding reference for the current pictureand/or one or more picture identifiers of respective referencepicture(s) as decoding reference for a subsequent, according to thedecoding order, picture of the video stream.

Step S32 preferably updates the decoded picture buffer to comprise therespective reference picture identified by the at least one pictureidentifier determined in step S31.

As previously discussed herein, the buffer description information canbe provided in a slice header or another control information field ofthe encoded representation of the picture. In such a case, step S30 ofFIG. 9 comprises retrieving the buffer description information from aslice header of the encoded representation of the picture. A multi-slicepicture generally comprises multiple slice headers. In such a case eachslice header preferably comprises the same buffer descriptioninformation. It is then sufficient in step S30 to retrieve the bufferdescription information from the first slice header of the picture asany remaining slice headers of the picture will include the same bufferdescription information. The buffer description information in the otherslice headers can then be used for error resilience if the first sliceis lost.

The buffer description information could explicitly include the pictureidentifier(s) of the reference picture(s). In such a case, step S30simply retrieves the at least one picture identifier from the bufferdescription information.

In an alternative embodiment, the buffer description informationcomprises delta values or delta identifiers. FIG. 10 is a flow chartillustrating an embodiment of step S31 in FIG. 9 for such a case. Themethod then continues from step S30 of FIG. 9 . A next step S40retrieves a respective delta identifier based on the buffer descriptioninformation. The delta identifier(s) is(are) used in step S41 togetherwith a picture identifier of the current picture to calculate thepicture identifier(s) of the reference picture(s). The method thencontinues to step S32 of FIG. 10 .

Thus, in this embodiment information available for the current pictureis used by the decoder to construct the final buffer description for thecurrent picture from the signaled buffer description information. Suchinformation includes, but is not limited to, current POC (POC(curr)),which together with a signaled deltaPOC can be used to calculate the POCof the reference picture (POC(ref)) as POC(ref)=POC(curr)+deltaPOC.

Another embodiment relates to using multiple predefined bufferdescriptions. FIG. 11 schematically illustrates such an approach. In afirst step S50 a data structure comprising multiple predefined bufferdescriptions is retrieved. In a preferred embodiment, the data structureis retrieved based on information carried in the encoded representationof the picture. For instance, the data structure can be signaled fromthe encoder side to the decoder side as a part of the PPS or the SPS ofthe encoded representation of the picture. In such a case, the datastructure is retrieved from the PPS or the SPS in step S50 based on aPPS identifier or SPS identifier used as part of the buffer descriptioninformation in the encoded representation of the picture, such as in theslice header. Alternatively, a PPS identifier is retrieved from theslice header and the PPS in turn comprises an SPS identifier that can beused if the data structure is signaled as part of the SPS.

A next step S51 retrieves an identifier of a buffer description as partof the buffer description information from the encoded representation ofthe picture, such as from the slice header. This identifier is employedin order to identify which predefined buffer description from the datastructure retrieved in step S50 to use for the current picture in stepS52. The method then continues to step S31 of FIG. 1 , where pictureidentifiers are determined from the identified predefined bufferdescription.

Using the embodiments it is possible to apply the optimal buffer schemefor any coding structure with very little overhead. What is needed inthe slice header is simply a reference to the correct buffer descriptionin the PPS or similar. Note that the buffer description can also be putin a picture header or similar, or a parameter set shared between allslices in a picture. The important property is that the bufferdescription to be used to decode a specific picture is sent with thepicture and not with the previous picture in decoding order as in theprior art. In addition, the buffer description information is used tosignal all reference pictures in the decoded picture buffer that shouldbe kept for decoding each respective picture in the video stream insteadof signaling delta information that can cause long-term mismatchesbetween the encoder side and the decoding side when a single data packetis unintentionally lost.

As previously disclosed herein, the explicit signaling of bufferdescription in the encoded representation of a picture and referencesignaling to predefined buffer description in a data structure can becombined. One way to combine the explicit signaling and the referencesignaling is to join the set of reference pictures described by explicitsignaling with the set of reference pictures described by referencesignaling to form a joint set of reference pictures. Then one or morereference pictures included in the set of reference pictures describedby reference signaling but not included in the explicit signaling arepossibly removed from the joint set of reference pictures in order tohave a final buffer description with no more than a maximum number(max_num_ref_frames) of reference pictures. Preferably pictures areremoved in the order that they are listed in the general bufferdescription, i.e. the predefined buffer description, starting with thelast one going forward.

FIG. 12 is a flow chart illustrating such an approach. The methodcontinues from step S30 in FIG. 9 and step S52 in FIG. 11 . Hence, inthis embodiment the buffer description information comprises both theidentifier of the buffer description retrieved in step S51 of FIG. 11and information defining at least one reference picture retrieved instep S30 of FIG. 9 . This information retrieved in step S30 could be theactual picture identifier(s) or the previously mentioned deltaidentifier(s).

A next step S60 determines a first set of at least one pictureidentifier identifying a respective reference picture from thepredefined buffer description identified in step S52 in FIG. 11 .Correspondingly, a second set of at least one picture identifieridentifying a respective reference picture is determined in step S61from the information retrieved in step S30 of FIG. 9 . Steps S60 and S61could be performed sequentially in any order or at least partly inparallel.

The following step S62 forms a joint set of picture identifiers based onthe first set determined in step S60 and the second set determined instep S61.

An optional but preferred following step S63 compares the total numberof picture identifiers (IDS) in the joint set with a maximum number ofreference pictures (MAX), i.e. the parameter max_num_ref_framestypically signaled in the SPS. If the total number of pictureidentifiers in the joint set exceeds the maximum number of referencepictures that can be stored in the decoded picture buffer the methodcontinues to step S64. This step S64 removes one or more pictureidentifiers included in the first set but not in the second set untilthe total number of picture identifiers in the joint set no longerexceeds the maximum number of reference pictures. Thus, the joint set isthereby updated by removing picture identifiers determined in step S60.The picture identifiers are preferably removed in the order that theyare listed in the predefined buffer description identified in step S52of FIG. 11 starting from the last picture identifier and going forward.

In a particular embodiment, the encoded representation of the picturepreferably comprises a flag to indicate whether explicit signaling ofthe buffer description information and/or implicit signaling of thebuffer description information has been selected for the currentpicture. In such a case, the decoder retrieves the flag from the encodedrepresentation of the picture, such as from the slice header or fromsome other control information field, in order to determine which typeof signaling of the buffer description information that has been usedfor the current picture.

FIG. 13 is a flow chart illustrating additional steps of the method inFIG. 9 , whereby the decoded picture buffer updated in step S32 is usedfor decoding purposes.

Once the decoded picture buffer has been updated in step S32 picturesmay be available in the decoded picture buffer but are not included inthe buffer description. Hence, in an embodiment, pictures that areavailable in the decoded picture buffer but not included in the bufferdescription are removed by the decoder from the decoded picture bufferor marked as “unused for reference” or “unused for prediction”. Thus, inthis embodiment removing reference pictures from the decoded picturebuffer or marking of pictures as “unused for reference” is performed bythe decoder prior to the picture decoding process of the picturecontaining the buffer description.

Step S70 illustrates this procedure by removing, from the decodedpicture buffer, any reference picture stored in the decoded picturebuffer and not being associated with any of the at least one pictureidentifier determined from the buffer description information. In analternative embodiment of this step S70 the reference picture(s) is(are)not necessarily removed from the decoded picture buffer. This embodimentof step S70 marks any reference picture stored in the decoded picturebuffer and not being associated with any of the at least one pictureidentifier determined from the buffer description information as unusedfor reference or unused for prediction. This means that the markedreference picture will not be used as prediction basis for the currentpicture or for any subsequent picture in the video stream. The decodercan therefore output the marked reference picture if there is a need toget an available position in the decoded picture buffer. In a particularembodiment, a picture marked as unused for reference cannot be includedin the buffer description and be re-marked as used for reference.

In a particular embodiment, any reference picture present in the decodedpicture buffer and associated with any of the picture identifiersdetermined based on the buffer description information is preferablymarked in step S70 as used for reference. In an embodiment, a referencepicture could be marked as either used for short-term reference or usedfor long-term reference. The selection between these two particularalternatives is preferably performed based on the buffer descriptioninformation.

Step S71 illustrates an additional optional embodiment. This embodimentoutputs zero or more of the pictures that are marked as unused forreference by the decoder according to the buffer description for displayby the decoder. One such example process for output is the bumpingprocess from H.264/MPEG-4 AVC. Output refers herein to output fordisplay. What pictures to use as reference pictures and what pictures tooutput, i.e. display, is separated in H.264 and HEVC. This means that apicture can be output before it is removed as reference picture, i.e.marked as unused for reference, or it can be removed as reference frameby marking it as unused for reference before it is output.

During the decoding process, it may happen that the buffer descriptioncontains information about pictures (also referred to as frames) thatare not available and therefore can not be used for reference pictures.In an embodiment, a number of “non-existing” pictures is generated bythe decoder if the buffer description contains information aboutpictures that are not available in the decoded picture buffer. Each suchpicture can be given values to variables holding information that isused by the decoder in the decoding process even if the picture is notused for inter prediction or motion vector prediction. Such informationcould include, but is not limited to, decoding order number, displayorder number, temporal layer information, view information, i.e.parameters such as frame_num, POC, temporal_id and view_id.

In an embodiment, the generation of non-existing pictures is performedby the decoder prior to the picture decoding process of the picturecontaining the buffer description information.

Steps S72 and S73 illustrate such an embodiment. Step S72 compares theat least one picture identifier determined from the buffer descriptioninformation with picture identifiers associated with reference picturesalready stored in the decoded picture buffer. If a determined pictureidentifier is not found among the picture identifiers of the referencepictures stored in the decoded picture buffer the picture associatedwith the determined picture identifier is determined to be missing ornon-existing. In an embodiment, a non-existing picture is optionallygenerated in step S73 and stored in the decoded picture buffer. Thisnon-existing picture is then assigned any parameters, such as pictureidentifier, obtained from the buffer description information with regardto this particular picture. The method then continues to step S33 ofFIG. 9 where the decoding process can be started.

Pictures that are signaled in the buffer description but do not exist inthe decoded picture buffer are preferably marked as “non-existing”.However since the POC and the temporal_id of such pictures are includedin the buffer description, no explicit signaling of reference picturelist modification is required. If the “non-existing” picture is not usedfor reference, the decoding process will proceed correctly without extrasignaling (and without the overhead incorporated with signalingreference picture list modifications explicitly).

In an embodiment, the marking of zero or more pictures in step S70 isperformed by the decoder in a first step. In a second step, zero or morepictures are output by the decoder in step S71. In a third step zero ormore “non-existing” pictures are generated by the decoder in step S73.

In an alternative embodiment, the marking of zero or more pictures isperformed by the decoder in a first step in step S70. Then an iterativeprocess of outputting pictures (S71) and generating non-existingpictures (S73) is performed by the decoder for each picture described inthe buffer description that is not available in the decoded picturebuffer.

The embodiments are strengthened further by applying buffer operations,e.g. the picture marking process, before the picture decoding of thecurrent picture, thus, having the explicit buffer description to applyto the current picture. This makes reference picture management evenless vulnerable to errors, improves the possibilities for temporalscalability and reduces the overhead introduced by reference picturelist modification signaling.

In an embodiment, the buffer description may contain information that isused by the decoder in reference picture list initialization orreference picture list modification or reference picture listcombination. An example is the order in which the pictures are listed ina buffer description can be used as the initial order for one of thereference picture lists in reference picture list initialization. Hence,the buffer description information can be used when the referencepicture list is created.

FIG. 14 is a flow chart illustrating such an approach. The methodcontinues from step S32 of FIG. 9 . A next step S80 performs referencepicture list initialization based on the buffer description information.In a particular embodiment of step S80, the reference picture listinitialization is performed based on the buffer description informationby ordering reference pictures in a reference picture list according toan order that the buffer description information defines the at leastone picture identifier determined in step S31 of FIG. 9 .

In an embodiment, restrictions are specified for the pictures includedin the buffer description. An example restriction is that a picture Awith temporal_id tId(A) described in the buffer description for apicture B with temporal_id tId(B) may not be a non-existing picture iftId(A)<tId(B).

In an embodiment it is also possible to signal changes to the propertiesof pictures in the decoded picture buffer through buffer descriptions.An example is to change temporal layer and/or display order of picturesthrough signaling the new value for temporal layer and/or display orderin the buffer description.

FIG. 16 is a simplified flow chart of a reference buffer schemeaccording to an embodiment. In this scheme all decoded picture bufferoperations are applied after parsing of the first slice header of apicture but before the picture decoding, using a description of thedecoded picture buffer as illustrated in FIG. 16 . The bufferdescription is, for instance, signaled in the slice header eitherexplicitly or by reference to a predefined structure signaled in a PPS.

The embodiments thereby provide large conceptual changes to the decodingprocess. In traditional H.264/MPEG-4 AVC and current design of HEVC,relative operations are given to the decoder either implicitly, i.e.sliding window, or explicitly, MMCO, and the decoder is responsible forapplying these relative operations and keeping track of the referencepictures, i.e. which pictures can be used for reference. In the proposedscheme the reference pictures, i.e. which pictures can be used forreference, is signaled within the current picture, such as in the sliceheader, thus removing the need of implicitly and explicitly signaledrelative operations.

This means that each picture will have an absolute description of thereference pictures instead of a relative description as in H.264/MEPG-4AVC where delta information is retrieved from MMCO or from using thesliding window process.

According to a particular embodiment, the buffer description containsdelta_POC, temporal_id and additional_picture_id of all referencepictures in the decoded picture buffer in order to provide an absolutereference to the pictures to be used as reference pictures. Thedelta_POC is used to calculate the POC of a reference picture asPOC(ref)=POC(current)+delta_POC. Pictures will, in an embodiment, beidentified by the pair POC and additional_picture_id. Temporal_id isincluded in the buffer description to enable correct reference picturelist modification in the case of lost or removed pictures, e.g. temporalscalability. The scheme is, though, not restricted to the codewordsdelta_POC, temporal_id and additional_picture_id. Any codeword that isassociated with a picture and used in the reference picture handling canbe used as picture identifier and may be included in the bufferdescription, either relative to the value of the current picture, e.g.POC and delta_POC, or absolute, e.g. temporal_id.

All pictures in the decoded picture buffer that are not part of thebuffer description are preferably marked as unused for reference.

In H.264/MPEG-4 AVC the process that delivers pictures for output(referred to as “bumping” process in FIG. 1 ) is sometimes performedprior to decoding, i.e. if there was a gap in frame_num. The “bumping”process is also performed after decoding and picture marking.

In the proposed scheme of FIG. 16 the “bumping” process is applied priorto decoding. It could be argued that this imposes extra delay in thedecoding process before delivery of pictures for output. However itshould be noted that the first picture to display is uniquely definedalready after the decoding process step as soon as the number ofnon-displayed pictures in decoded picture buffer is larger than or equalto num_reorder_frames. Thus, a decoder can deliver that picture fordisplay directly after the decoding process step. Thus the delay of theproposed scheme is equal to the delay of the current HEVC scheme.

In H.264/MPEG-4 AVC the syntax element frame_num is used to identifypictures in the decoded picture buffer and to detect gaps in frame_num.If gaps_in_frame_num_allowed is equal to 1 the decoder shall insert“non-existing” frames in the decoded picture buffer in order for thesliding window process to operate correctly.

In the proposed scheme illustrated in FIG. 16 the combination of POC andadditional_picture_id can be used to identify pictures in the decodedpicture buffer. The proposed scheme does not need to contain a slidingwindow process. Therefore, it is proposed to remove the syntax elementsframe_num and gaps_in_frame_num_allowed.

FIG. 15 is an example of a coding structure to which the embodiments canbe applied. In the example in FIG. 15 , the second picture in decodingorder will include POC in its buffer description while the third picturein decoding order will include POC 0 and POC 4 in its bufferdescription. The fourth picture in decoding order must include not onlyPOC 0 and POC 2 in its buffer description but also POC 4 since thatpicture will be used for reference in the future. The fifth picture inthe decoding order does not need to include POC 0 in its bufferdescription unless it is going to be used for reference in the future.If POC 0 is not included in the buffer description it will be madeunused for reference.

In the example in FIG. 15 additional_picture_id (or additional_id) is 0for all pictures. It is recommended to set additional_picture_id to 0unless there are two different pictures available for reference with thesame POC, due to POC wrap-around. If POC is the same for two differentpictures the additional_picture_id is preferably different in order tonot unconditionally replace the older picture with the newer one.Additional_picture_id can be used in order to provide all thefunctionalities and possibilities that are provided by long termpictures in H.264/MPEG-4 AVC. Hence, in a particular embodiment theadditional_picture_id constituting a part of the picture identifiercould be used to signal whether a reference picture is to be used forlong-term reference or for short-term reference.

Consider an example where an encoder would like to use a picture A withPOC 14 and additional_picture_id set to 0, as a “long term picture”. Theencoder must then make sure that the picture A is included in the bufferdescription in the slice header of all pictures following A for as longas the encoder wants picture A to be available for reference. If theencoder would like to encode another picture with POC 14 (due to POCwrap-around) while A is still available for reference, the encodershould select another value of additional_picture_id, for example 1, toavoid that A is replaced as a reference picture.

For pictures that will be used for reference for a short period of time(short term reference pictures) it is recommended to setadditional_picture_id to 0, in order to minimize the bit overhead. Forpictures that will be used for reference for a long period of time (longterm pictures) it is recommended that the encoder makes sure that no twopictures with the same POC and the same additional_picture_id areavailable for prediction at the same time.

A further aspect of the embodiments defines an encoder configured tocreate buffer description information defining which pictures to be usedfor reference pictures, i.e. stored in a decoded picture buffer to beused for subsequent decoding. At least part of the buffer descriptioninformation is inserted in the bitstream encoded by the encoder.

FIG. 18 is a schematic block diagram of an embodiment of an encoder 100.The encoder 100 is configured to encode a picture of a video streamcomprising multiple pictures. The encoder 100 comprises a referencepicture determiner 110 configured to determine at least one referencepicture for the current picture among the multiple pictures of the videostream. The at least one reference picture is used as encoding referencefor the current picture. A picture identifier provider 120 of theencoder 100 is configured to provide a respective picture identifier ofeach reference picture determined by the reference picture determiner110. A picture identifier provided by the picture identifier provider120 identifies its associated reference picture. The pictureidentifier(s) provided by the picture identifier provider 120collectively constitute a buffer description for the current picture bylisting picture identifier(s) of the reference picture(s) required forencoding and decoding the current picture and optionally also anyprevious reference picture(s) of the video stream required for encodingand decoding a subsequent picture of the video stream.

The encoder 100 also comprises a buffer description informationgenerator 130 configured to generate information of the bufferdescription, i.e. the buffer description information, defining the atleast one reference picture determined by the reference picturedeterminer 110. The buffer description information generator 130 isconfigured to generate this buffer description information based on theat least one picture identifier from the picture identifier provider120.

A data inserter 140 is implemented in the encoder 100 to insert thebuffer description information generated by the buffer descriptioninformation generator 130 into an encoded representation of the currentpicture. Hence, the coded bitstream of the video stream with regard tothe current picture carries the buffer description information. Thismeans that the encoded representation of the picture will carry thebuffer description information that defines the relevant information,i.e. picture identifiers, required by a decoder in order to update thedecoded picture buffer for the current picture to thereby enabledecoding of the encoded representation of the picture.

As has been previously discussed herein the reference picture determiner110 is preferably configured to determine any reference picture of thevideo stream as encoding reference for not only the current picture butalso any reference picture, preferably present in the video stream priorto the current picture in the decoding order, as encoding reference forone or more subsequent pictures according to the decoding order.

In a particular embodiment each picture can be composed of one or moreslices. Each such slice may then be encoded and decoded independently ofother slices of the same picture. Hence, the reference picturedeterminer 110 preferably determines, for at least one slice, preferablyfor each slice of the picture, at least one reference picture to be usedfor encoding and decoding the particular slice. The picture identifierprovider 120 preferably provides respective picture identifiers for allreference pictures determined by the reference picture determiner 110for the current picture, i.e. for all its slices. The buffer descriptioninformation is thereby generated by the buffer description informationgenerator 130 based on these provided picture identifiers. In apreferred embodiment, the data inserter 140 is configured to insert thebuffer description information in a respective slice header of theencoded representation of the picture. In such a case, each slice headerof the current picture preferably carries the same buffer descriptioninformation. This enables decoding of a given slice even, though,another slice of the picture is unintentionally lost.

Also other positions among control information in the encodedrepresentation of the picture are possible for carrying the bufferdescription information as previously discussed herein.

The buffer description of the embodiments could include the pictureidentifier(s) of the reference picture(s). Alternatively, the bufferdescription comprises the previously discussed delta identifier(s) thatcan be used together with the picture identifier of the current pictureto calculate the picture identifier(s) of the reference picture(s).

In such a case the encoder 100 preferably comprises an identifiercalculator 150 that is configured to calculate, for each pictureidentifier provided by the picture identifier provider 120, a differencebetween the picture identifier and the picture identifier of the currentpicture. This difference corresponds to the delta identifier for thereference picture. The buffer description information generator 130 isthen configured to generate the buffer description information based onthe at least one difference/delta identifier calculated by theidentifier calculator 150. The buffer description information therebydefines the at least one picture identifier of the reference picture(s)relative to the picture identifier of the current picture.

The encoded representation of the picture could carry the pictureidentifiers provided by the picture identifier provider 120 or the deltaidentifiers calculated by the identifier calculator 150 as bufferdescription information, for instance in the slice headers. Thisprovides an explicit signaling of the buffer description in the encodedrepresentation of the picture.

In an alternative embodiment, the encoder 100 comprises a data structuregenerator 160 configured to generate a data structure comprisingmultiple predefined buffer descriptions. Each such predefined bufferdescription thereby defines at least one reference picture. The datastructure can thereby be used for multiple pictures in the video streamfor the purpose of providing the buffer description to be used duringdecoding for the pictures. The buffer description information generator130 is thereby configured to select a buffer description of the datastructure based on the at least one picture identifier provided by thepicture identifier provider 120 for the current picture. The bufferdescription that correctly matches the at least reference pictureidentifier is thereby selected and the buffer description informationgenerated by the buffer description information generator 130 comprisesan identifier of the selected buffer description.

The data structure generated by the data structure generator 160 couldthen be inserted in a control information field associated with anencoded representation of the video stream, such as in a PPS or SPS. Anidentifier of the relevant control information, i.e. a PPS identifier oran SPS identifier (could be in the form of a PPS identifier to a PPSthat in turn comprises an SPS identifier to the relevant SPS), ispreferably included in control information of the encoded representationof the current picture, such as in the slice header. In an embodiment,the slice header or other control information field thereby carries thecontrol information identifier and the identifier of the selected bufferdescription as the buffer description information.

The above description embodiments of explicit signaling of pictureidentifiers and reference signaling of picture identifiers can becombined. In such a case, the buffer description information generator130 is configured to generate the buffer description informationcomprising the identifier of the selected buffer description, preferablyalso the control information identifier, and comprising information,such as delta identifier or explicit picture identifier, defining apicture identifier of a reference picture.

The encoder 100 could use any of the previously discussed examples ofpicture identifiers, such as POC value that unambiguously identifiers areference picture, or a POC value and an additional identifier.

The video stream could be a scalable video stream where the pictures aregrouped into multiple layers. In such a case the picture identifierprovider 120 is configured to provide, for each reference picture, thepicture identifier and temporal layer information or identifieridentifying a layer to which the reference picture belongs. The bufferdescription information generator 130 then generates the bufferdescription information based on the at least one picture identifier andbased on the temporal layer information. Thus, the buffer descriptioninformation preferably defines the at least one picture identifier andthe temporal layer information.

The reference picture determiner 110 is preferably configured todetermine the at least one reference picture as encoding reference forinter prediction and/or motion vector prediction for the currentpicture. The at least one reference picture can thereby be determined bymaximizing the compression or coding efficiency of the encodedrepresentation of the picture, i e minimizing the number of symbols,such as bits, required for representing the encoded representation. Thiscompression maximization is preferably performed while complying with atleast one side constraint selected from having a predefined maximumnumber of reference pictures and generating a decodable representationof the video stream also when at least one encoded representation of apicture is removed from the encoded representation of the video stream.A further alternative or additional side constraint is that a selectedreference picture must be available for reference and prediction in thedecoded picture buffer.

The encoder could be implemented at least partly in software. In such anembodiment as shown in FIG. 19 , the encoder 300 comprises an inputsection 310 configured to receive multiple pictures of a video stream.The encoder 300 also comprises a processor 330 configured to processcode means of a computer program stored in a memory 340. The code meanscauses, when run on the processor 330, the processor 330 to determine,for a picture of the video stream, at least one reference picture of thevideo stream as encoding reference for the picture. The processor 330 isalso caused to provide, for each reference picture, a picture identifieridentifying the reference picture and generate, based on the at leastone picture identifier, buffer description information defining the atleast one reference picture. The processor 330 is further caused toinsert the buffer description information in an encoded representationof the picture. The encoder 300 also comprises an output section 320configured to output the encoded representations of the pictures.

The processor 330 could be a general purpose or specially adaptedcomputer, processor or microprocessor, such as a central processing unit(CPU). The software includes computer program code elements or softwarecode portions effectuating the operation of at least the referencepicture determiner 110, the picture identifier provider 120, the bufferdescription information generator 130, and the data inserter 140 of FIG.18 .

The program may be stored in whole or part, on or in one or moresuitable volatile computer readable media or data storage means, such asRAM, or one or more non-volatile computer readable media or data storagemeans, such as magnetic disks, CD-ROMs, DVD disks, hard discs, in ROM orflash memory. The data storage means can be a local data storage meansor is remotely provided, such as in a data server. The software may thusbe loaded into the operating memory of a computer or equivalentprocessing system for execution by a processor. The computer/processordoes not have to be dedicated to only execute the above-describedfunctions but may also execute other software tasks. A non-limitingexample of program code used to define the encoder 300 include singleinstruction multiple data (SIMD) code.

Alternatively, the encoder can be implemented in hardware. There arenumerous variants of circuitry elements that can be used and combined toachieve the functions of the units 110-160 of the encoder 100 in FIG. 18. Such variants are encompassed by the embodiments. Particular examplesof hardware implementation of the encoder 100 is implementation indigital signal processor (DSP) hardware and integrated circuittechnology, including both general-purpose electronic circuitry andapplication-specific circuitry.

According to an aspect of the embodiments a transmitter 200 as shown inFIG. 17 is provided. The transmitter 200 comprises an input section 210configured to receive multiple pictures 10 of a video stream. Thepictures 10 are forwarded to an encoder 100, such as illustrated in FIG.18 or in FIG. 19 , which is configured to encode the multiple pictures10 to generate respective encoded representations of the multiplepictures. An output section 220 of the transmitter 200 is configured tooutput the respective encoded representations of the multiple picturesas a coded bitstream carrying the buffer description information of theembodiments.

An aspect of the embodiments relates to a decoder for detecting bufferdescription information and for determining absolute reference topictures to be used as reference pictures for decoding based on thedetected buffer description and a buffer for storing the pictures to beused as reference pictures.

FIG. 21 is a schematic block diagram of a decoder 400 according to anembodiment. The decoder 400 is configured to decode an encodedrepresentation of a picture of a video stream comprising multiplepictures. The decoder 400 comprises a data retriever 410 configured toretrieve buffer description information defining at least one referencepicture from the encoded representation of the picture. The bufferdescription information is used by a picture identifier determiner 420that is configured to determine at least one picture identifieridentifying a respective reference picture as decoding reference for thepicture. The decoder 400 also comprises a buffer manager 430 configuredto update a decoded picture buffer of or associated to the decoder 400based on the at least one picture identifier determined by the pictureidentifier determiner 420.

In a particular embodiment, the data retriever 410 is configured toretrieve the buffer description information defining multiple pictureidentifiers from the encoded representation of the picture. The pictureidentifier determiner 420 is then configured to determine, based on theretrieved buffer description information, at least one pictureidentifier identifying a respective reference picture as decodingreference for the picture and at least one picture identifieridentifying a respective reference picture as decoding reference for asubsequent, according to a decoding order, picture of the video stream.

The buffer manager 430 is preferably configured to update the decodedpicture buffer to comprise the respective reference picture identifiedby the at least one picture identifier.

Thus, the buffer description information carried by the encodedrepresentation of the picture is used to identify which referencepicture(s) that need to be stored in the decoded picture buffer fordecoding the current picture and for decoding a subsequent picture ofthe video stream. The buffer description information thereby providesthe information to the decoder 400 required to update the decodedpicture buffer to store the correct reference picture(s).

Once the buffer manager 430 has updated the decoded picture buffer thedecoder 400 can decode the picture based on the encoded representationof the picture and at least one reference picture stored in the updateddecoded picture buffer.

The buffer description information is preferably provided in controlinformation of the encoded representation of the picture. For instance,the data retriever 410 could be configured to retrieve the bufferdescription information from a slice header of the encodedrepresentation of the picture. In such a case the buffer descriptioninformation is preferably retrieved from the first slice header receivedfor the current picture since any remaining slice headers of the picturewill preferably carry the same buffer description information.

The retrieved buffer description information could include explicitpicture identifiers of the reference pictures to be stored in thedecoded picture buffer. In an alternative embodiment, the bufferdescription information defines a respective delta identifier for thereference pictures. The picture identifier determiner 420 is thenconfigured to retrieve the at least one delta identifier from the bufferdescription information and calculate the at least one pictureidentifier based on the respective delta identifier and the pictureidentifier of the current picture, preferably as a sum of the deltaidentifier and the picture identifier of the current picture.

Instead of explicit signaling of picture identifiers or deltaidentifiers in the encoded representation of the picture a referencesignaling can be used. The data retriever 410 is in this embodimentconfigured to retrieve an identifier of a buffer description from theencoded representation of the picture. The decoder 400 preferablycomprises a buffer description identifier 480 configured to identify abuffer description from a data structure comprising multiple predefinedbuffer descriptions using the retrieved identifier of the bufferdescription.

The data retriever 410 is preferably in this embodiment also configuredto retrieve the data structure defining the multiple predefined bufferdescriptions from a control information field of or associated with anencoded representation of the video stream, such as from a PPS or SPS.

In a particular embodiment a control information field of the encodedrepresentation of the picture, such as slice header, preferablycomprises an identifier of the control information field, such as PPS orSPS, carrying the data structure. The data retriever 410 therebyretrieves this identifier and uses it to identify the relevant controlinformation field with the data structure.

In another embodiment the buffer description information comprises theidentifier of the buffer description and information, such as pictureidentifier or delta identifier, identifying at least one pictureidentifier. The picture identifier determiner 420 is then configured todetermine a first set of at least one picture identifier from theidentified buffer description, preferably from the data structure. Thepicture identifier determiner 420 also determines a second set of atleast one picture identifier from the information defining at least onepicture identifier, where this information has been explicitly signaledin the encoded representation of the picture, such as in the form of apicture identifier or a delta identifier. A joint set of pictureidentifiers is then formed from the first set and the second set by thepicture identifier determiner 420.

In a particular embodiment the decoder 400 comprises a number comparator440 configured to compare a total number of picture identifiers in thejoint set with a maximum number of reference pictures that can be storedin the decoded picture buffer. If the total number of pictureidentifiers in the joints set exceeds the maximum number of referencepictures the picture identifier determiner 420 is configured to removeone or more picture identifiers included in the first set but not in thesecond set until the total number of picture identifiers in the jointset no longer exceeds the maximum number of reference pictures.

The buffer manager 430 of decoder 400 is configured, in a particularembodiment, to remove any reference picture present in the decodedpicture buffer but not being associated with any of the at least onepicture identifier determined from the buffer description information.Hence, any reference picture that is stored in the decoded picturebuffer but not identified from the buffer description information ispreferably removed from the decoded picture buffer by the buffer manager430.

In an alternative and preferred approach, the buffer manager 430 isconfigured to mark all reference pictures stored in the decoded picturebuffer but not being associated with any of the at least one pictureidentifier from the buffer description information as unused forreference, also referred to as unused for prediction.

In a particular embodiment, any reference picture present in the decodedpicture buffer and associated with any of the picture identifiersdetermined based on the buffer description information is preferablymarked by the buffer manager 430 as used for reference. In anembodiment, a reference picture could be marked as either used forshort-term reference or used for long-term reference. The selectionbetween these two particular alternatives is preferably performed basedon the buffer description information.

The buffer manager 430 of the decoder 400 is preferably configured tomark any reference picture prior to the decoder 400 decoding the currentpicture.

In a particular embodiment, the decoder 400 comprises an outputtingsection 450 configured to output zero or more pictures from the decodedpicture buffer for display prior to the decoder 400 decodes the currentpicture. In a particular embodiment, the outputting section 450 outputsany reference picture marked as unused for reference by the buffermanager 430.

An optional embodiment of the decoder 400 comprises an identifiercomparator 460 configured to compare the at least one picture identifierdetermined by the picture identifier determiner 420 with the pictureidentifier(s) of reference picture(s) stored in the decoded picturebuffer. If any of the at least one picture identifier defined by thebuffer description information does not have a matching referencepicture in the decoded picture buffer the picture associated with thegiven picture identifier is determined by the picture identifierdeterminer 420 to be non-existing or missing.

In an optional embodiment, the picture identifier determiner 420 isconfigured to generate any non-existing picture and store such agenerate picture in the decoded picture buffer. The picture identifierand any other information obtainable from the buffer descriptioninformation with regard to the non-existing picture is preferablyassigned to the generated picture by the picture identifier determiner420.

The decoder 400 may also comprise a list manager 470 configured toperform reference picture list initialization based on the bufferdescription information. In a particular embodiment, the list manager470 is configured to perform the reference picture list initializationby ordering reference pictures in a reference picture list according toan order that the buffer description information defines the at leastone picture identifier. Hence, the buffer description information notonly defines the picture identifiers of the reference pictures but theorder at which these are defined in the buffer description informationalso provides instructions to the list manager 470 with regard toforming the reference picture list.

The decoder could be implemented at least partly in software. In such anembodiment as shown in FIG. 22 , the decoder 600 comprises an inputsection 610 configured to receive encoded representations of multiplepictures of a video stream. The decoder 600 also comprises a processor630 configured to process code means of a computer program stored in amemory 640. The code means causes, when run on the processor 630, theprocessor 630 to retrieve buffer description information defining atleast one reference picture from an encoded representation of a picture.The code means also causes the processor 630 to determine at least onepicture identifier identifying a respective reference picture from thebuffer description information. The respective reference picture is tobe used as decoding reference for the picture. The processor 630 isfurther caused to update a decoded picture buffer 650 based on the atleast one picture identifier. The decoder 600 also comprises an outputsection 620 configured to output the decoded pictures of the videostream.

The processor 630 could be a general purpose or specially adaptedcomputer, processor or microprocessor, such as a central processing unit(CPU). The software includes computer program code elements or softwarecode portions effectuating the operation of at least data retriever 410,the picture identifier determiner 420 and the buffer manager 430 of FIG.21 .

The program may be stored in whole or part, on or in one or moresuitable volatile computer readable media or data storage means, such asRAM, or one or more non-volatile computer readable media or data storagemeans, such as magnetic disks, CD-ROMs, DVD disks, hard discs, in ROM orflash memory. The data storage means can be a local data storage meansor is remotely provided, such as in a data server. The software may thusbe loaded into the operating memory of a computer or equivalentprocessing system for execution by a processor. The computer/processordoes not have to be dedicated to only execute the above-describedfunctions but may also execute other software tasks. A non-limitingexample of program code used to define the decoder 600 include singleinstruction multiple data (SIMD) code.

Alternatively, the decoder can be implemented in hardware. There arenumerous variants of circuitry elements that can be used and combined toachieve the functions of the units 410-480 of the decoder 400 in FIG. 21. Such variants are encompassed by the embodiments. Particular examplesof hardware implementation of the decoder 400 is implementation indigital signal processor (DSP) hardware and integrated circuittechnology, including both general-purpose electronic circuitry andapplication-specific circuitry.

According to an aspect of the embodiments a receiver 500 as shown inFIG. 20 is provided. The receiver 500 comprises an input section 510configured to receive encoded representations of multiple pictures of avideo stream. The encoded representation carries buffer descriptioninformation according to the embodiments. The encoded representationsare forwarded to a decoder 400, such as illustrated in FIG. 21 or inFIG. 22 , which is configured to decode the encoded representations ofthe multiple pictures. An output section 520 of the receiver 500 isconfigured to output decoded pictures of the video stream. The receiver500 also comprises a decoded picture buffer 530 storing referencepictures to be used by the decoder 400 when decoding the pictures.

ANNEX

The present annex presents a proposed syntax of an embodiment. In theannex added syntax element with regard to the prior art HEVC proposal ismarked with underlining and removed syntax elements are marked withstrikethrough.

SLICE HEADER Descriptor slice_header( ) {  first_tb_in_slice ue(v) entropy_slice_flag u(1)  if( !entropy_slice_flag ) {   slice_type ue(v)  pic_parameter_set_id ue(v)   

  additional_picture_id ue(v)   if( IdrPicFlag )    idr_pic_id ue(v)  if( pic_order_cnt_type = = 0 )    pic_order_cnt_lsb /* u(v)  if(!IdrPicFlag){    buffer_description_reference_flag u(1)   if(buffer_description_reference_flag = = 1){    buffer_description_id ue(v)    }   if(buffer_description_reference_flag = = 0){    number_of_explicitly_signaled_pictures ue(v)    for(number_of_explicitly_signaled_pictures) {     additional_picture_id ue(v)      if(num_reorder_frames > 0)      delta_POC_sign u(1)      absolute_delta_POC_minus_one ue(v)     temporal_id u(v)      }     }    }   }   if( slice_type = = P ||slice_type = = B ) {    num_ref_idx_active_override_flag u(1)    if(num_ref_idx_active_override_flag ) {     num_ref_idx_l0_active_minus1ue(v)     if( slice_type = = B )      num_ref_idx_l1_active_minus1 ue(v)   }   }   ref_pic_list_modification( )   ref_pic_list_combination( )   

   

  if( entropy_coding_mode_flag && slice_type != I)    cabac_init_idcue(v)   slice_qp_delta se(v)   if( adaptive_loop_filter_enabled_flag )   alf_param( )   if( deblocking_filter_control_present_flag ) {   disable_deblocking_filter_idc    if( disable_deblocking_filter_idc !=1 ) {     slice_alpha_c0_offset_div2     slice_beta_offset_div2    }   }  if( slice_type = = B )    collocated_from_l0_flag u(1)  } else   if(entropy_coding_mode_flag && slice_type != I)    cabac_init_idc ue(v) }

PICTURE PARAMETER SET Descriptor pic_parameter_set_rbsp( ) { pic_parameter_set_id ue(v)  seq_parameter_set_id ue(v) entropy_coding_mode_flag u(1)  num_ref_idx_l0_default_active_minus1ue(v)  num_ref_idx_l1_default_active_minus1 ue(v)  pic_init_qp_minus26/* relative to 26 */ se(v)  constrained_intra_pred_flag u(1) number_of_buffer_descriptions ue(v) for(number_of_buffer_descriptions){  number_of_pictures_in_buffer_description ue(v)  for(number_of_pictures_in_buffer_description) {   if(num_reorder_frames > 0)     delta_POC_sign u(1)   absolute_delta_POC_minus_one ue(v)    temporal_id u(v)   } rbsp_trailing_bits( ) }

Proposed Semantics Decoding Process for Buffer Description Application

This process is invoked at the beginning of the decoding process foreach I, P, or B picture, prior to decoding of the first Coding Unit inthe first slice of the picture and prior to the decoding process forreference picture list construction.

The process may result in marking one or more picture as “unused forreference”.

The process may result in the construction of “non-existing” pictures asdescribed in Creation of non-existing pictures.

A reference picture is identified for use in the decoding process by thevariables POC and additional_picture_id.

A reference picture r in the decoded picture buffer is said to be a partof a buffer description if and only if POC(r)=POC(curr)+delta_POC(i) andadditional_picture_id(r)=additional_picture_id(i) for any i in the rangeof 0 to number_pictures_in_buffer_description.

Sequence of Operation for Buffer Description Application

Buffer description application proceeds in the following ordered steps:

-   -   1. Depending on whether the current picture is an IDR picture,        the following applies.        -   If the current picture is an IDR picture, all reference            pictures are marked as unused for reference.        -   Otherwise (the current picture is not an IDR picture), the            following ordered steps applies:            -   I. The buffer description information in the slice                header of the first slice of the current picture is used                to create a list of reference pictures, referred to as                the buffer description as described in Creation of a                buffer description.            -   II. All reference pictures (in the decoded picture                buffer) that are not a part of the buffer description                are marked as “unused for reference”.            -   III. If one or more pictures that are included in the                buffer description are not available for reference (i.e.                existing in the reference buffer marked as “used for                reference”) the process described in Creation of                non-existing pictures is invoked.    -   2. The slices of the current picture are decoded.

Creation of a Buffer Description

If buffer_description_reference_flag is 1, a buffer description iscreated setting deltaPOC to the deltaPOC of the PPS buffer descriptionentry identified by buffer_description_id, temporal_id to thetemporal_id of the PPS buffer description entry identified bybuffer_description_id and additional_picture_id to 0 for each picture iin the range from 0 to number_of_pictures_in_buffer_description−1.

If buffer_description_reference_flag is 0, a buffer description iscreated setting deltaPOC to the deltaPOC of the explicitly signaledbuffer description, temporal_id to the temporal_id of the explicitlysignaled buffer description and additional_picture_id toadditional_picture_id of the explicitly signaled buffer description foreach picture i in the range from 0 tonumber_of_explicitly_signaled_pictures−1.

Creation of Non-Existing Pictures

This process is preferably invoked for each picture that is included inthe buffer description of the current picture but is not available inthe decoded picture buffer. That is if the buffer description of thecurrent picture contains an entry i, with a POC(i) andadditional_picture_id(i) such that there is no picture j in the decodedpicture buffer marked as “used for reference” with POC(j)==POC(i) andadditional_picture_id(j)==additional_picture_id(i).

For each such description a “non-existing” picture is generated with POCset to POC(i), additional_picture_id set to additional_picture_id(i),temporal_id set to temporal_id(i) and marked as “non-existing” and “usedfor reference”. The sample values of the generated frames may be set toany value. These generated frames which are marked as “non-existing”shall not be referred to in the inter prediction process.

The embodiments described above are to be understood as a fewillustrative examples of the present invention. It will be understood bythose skilled in the art that various modifications, combinations andchanges may be made to the embodiments without departing from the scopeof the present invention. In particular, different part solutions in thedifferent embodiments can be combined in other configurations, wheretechnically possible. The scope of the present invention is, however,defined by the appended claims.

What is claimed is:
 1. A method of decoding an encoded representation ofa picture in a video stream of multiple pictures, said methodcomprising: retrieving, using a decoder configured to decode saidencoded representation of said picture, buffer description informationdefining at least one reference picture, wherein said buffer descriptioninformation is retrieved from said encoded representation of saidpicture; and determining, based on said buffer description information,at least one reference picture identifier, said at least one referencepicture identifier identifying a respective reference picture among saidat least one reference picture, said respective reference picture beinga decoding reference for said picture, wherein determining said at leastone reference picture identifier comprises: (i) retrieving, based onsaid buffer description information, a respective delta identifier; and(ii) calculating said at least one reference picture identifier based ona picture identifier identifying said picture and said respective deltaidentifier.
 2. The method according to claim 1, further comprising:repeating the retrieving and the determining for other referencepictures; and marking as unused for reference, one of the otherreference pictures that is not identified by one of the referencepicture identifiers.
 3. The method according to claim 1, wherein:retrieving said buffer description information comprises retrieving saidbuffer description information defining multiple reference pictures fromsaid encoded representation of said picture; and determining said atleast one reference picture identifier comprises determining, based onsaid buffer description information, i) at least one picture identifieridentifying a respective reference picture among said multiple referencepictures as the decoding reference for said picture and ii) at least onepicture identifier identifying a respective reference picture among saidmultiple reference pictures as a decoding reference for a subsequent,according to a decoding order, picture in said video stream.
 4. Themethod according to claim 1, wherein delta display order information isincluded in the buffer description information and is encoded with avariable-length code.
 5. The method according to claim 1, whereindetermining said at least one reference picture identifier comprisesdetermining, based on said buffer description information, at least onepicture order count (POC) value identifying said respective referencepicture as the decoding reference for said picture.
 6. The methodaccording to claim 5, wherein determining said at least one POC valuecomprises determining, based on said buffer description information,said at least one picture order count (POC) value and at least oneadditional identifier identifying said respective reference picture asthe decoding reference for said picture.
 7. A decoder configured todecode an encoded representation of a picture in a video stream ofmultiple pictures, said decoder comprising: a data retriever configuredto retrieve buffer description information defining at least onereference picture, wherein said buffer description information isretrieved from said encoded representation of said picture; and apicture identifier determiner configured to determine, based on saidbuffer description information, at least one reference pictureidentifier, said at least one reference picture identifier identifying arespective reference picture among said at least one reference picture,said respective reference picture being a decoding reference for saidpicture, wherein said picture identifier determiner is configured to i)retrieve, based on said buffer description information, a respectivedelta identifier, and ii) calculate said at least one reference pictureidentifier based on a picture identifier identifying said picture andsaid respective delta identifier.
 8. The decoder according to claim 7,wherein: the data retriever and picture identifier operate to repeat theretrieving and the determining for other reference pictures; and thepicture identifier is further configured to mark as unused forreference, one of the other reference pictures that is not identified byone of the reference picture identifiers.
 9. The decoder according toclaim 7, wherein: said data retriever is configured to retrieve saidbuffer description information defining multiple reference pictures fromsaid encoded representation of said picture; and said picture identifierdeterminer is configured to determine, based on said buffer descriptioninformation, i) at least one picture identifier identifying a respectivereference picture among said multiple reference pictures as the decodingreference for said picture and ii) at least one picture identifieridentifying a respective reference picture among said multiple referencepictures as a decoding reference for a subsequent, according to adecoding order, picture in said video stream.
 10. The decoder accordingto claim 7, wherein said buffer manager is configured to mark allreference pictures present in said decoded picture buffer and not beingassociated with any of said at least one reference picture identifierdetermined based on said buffer description information as unused forreference.
 11. The decoder according to claim 10, wherein said buffermanager is configured to mark any reference picture prior to saiddecoder decodes said picture.
 12. The decoder according to claim 7,wherein delta display order information is included in the bufferdescription information and is encoded with a variable-length code. 13.A receiver comprising: an input section configured to receive encodedrepresentations of multiple pictures in a video stream; a decoderaccording to claim 7, configured to decode said encoded representationsof said multiple pictures; and an output section configured to outputdecoded pictures of said video stream.
 14. A decoder comprising: aninput section configured to receive encoded representations of multiplepictures in a video stream; a processor; and a memory coupled to saidprocessor and comprising instructions stored in said memory, saidprocessor configured to process said instructions, wherein saidinstructions, when run on said processor, cause said processor to:retrieve buffer description information defining at least one referencepicture, wherein said buffer description information is retrieved froman encoded representation of a picture; determine, based on said bufferdescription information, at least one reference picture identifier, saidat least one reference picture identifier identifying a respectivereference picture among said at least one reference picture, saidrespective reference picture being a decoding reference for saidpicture, wherein determining said at least one reference pictureidentifier comprises: retrieving, based on said buffer descriptioninformation, a respective delta identifier; and calculating said atleast one reference picture identifier based on a picture identifieridentifying said picture and said respective delta identifier; and anoutput section configured to output decoded pictures of said videostream.
 15. The decoder according to claim 14, wherein the instructionsfurther cause the processor to: repeat the retrieving and thedetermining for other reference pictures; and mark as unused forreference, one of the other reference pictures that is not identified byone of the reference picture identifiers.
 16. The decoder according toclaim 14, wherein: retrieving said buffer description informationcomprises retrieving said buffer description information definingmultiple reference pictures from said encoded representation of saidpicture; and determining said at least one reference picture identifiercomprises determining, based on said buffer description information, i)at least one picture identifier identifying a respective referencepicture among said multiple reference pictures as the decoding referencefor said picture and ii) at least one picture identifier identifying arespective reference picture among said multiple reference pictures as adecoding reference for a subsequent, according to a decoding order,picture in said video stream.
 17. The decoder according to claim 14,wherein delta display order information is included in the bufferdescription information and is encoded with a variable-length code. 18.The decoder according to claim 14, wherein determining said at least onereference picture identifier comprises determining, based on said bufferdescription information, at least one picture order count (POC) valueidentifying said respective reference picture as the decoding referencefor said picture.
 19. The decoder according to claim 18, whereindetermining said at least one POC value comprises determining, based onsaid buffer description information, said at least one picture ordercount (POC) value and at least one additional identifier identifyingsaid respective reference picture as the decoding reference for saidpicture.